This invention relates to guidance systems and is particularly concerned with systems in which a moving body is to be guided to a predetermined location. One method of enabling a body to determine its position is by allowing it to view its surroundings and to compare its field of view with a reference pattern. The process of identifying the viewed scene with a reference pattern is often termed correlation. In principle, correlation, whether it is of a one or two dimensional nature, involves searching for a particular signal or data pattern within a temporal or spatial window in another signal or data pattern. In the present invention, the signals or data patterns represent two dimensional images and thus one image is being searched for within another image, i.e. the two images are of different sizes, one being a sub-set of the other.
This aspect of correlation is sometimes termed "scene imaging by area correlation" and it can be used to bring a moving body to a predetermined location and to cause it to follow a predetermined path. It is merely necessary for the body to view its surroundings as it moves, and to compare the viewed scene with reference data relating to localised reference areas along its path. As the viewed scene is periodically correlated with the localised reference areas, the moving body can determine whether it is on the required path or whether it diverges from it, and in the latter case correction can be applied.
It is customary for the reference areas to be very much larger than the field of view so that the guidance system has the task of searching through a relatively large reference area to determine whether any portion of it correlates with its current field of view. A relatively large area is usually used since one can fairly be certain that it will contain a large number of well distributed prominent visual features of a distinctive nature, and furthermore, such an arrangement is able to tolerate relatively large positional errors associated with the movement of the body. Thus even if the body is laterally displaced by a significant extent from the required path, its position can still be determined providing that its field of view is within the boundaries of the relatively large reference area.
However, such an arrangement is very difficult to implement into a practical system, and moreover if the positional error should be so large that the optical field of view does not fall within the single reference area, guidance of the body is not possible.